1. Field of the Invention
The present invention relates to a steam turbine.
2. Description of Related Art
In the last stage or a stage one or two stages therebefore of a low pressure turbine, pressure is generally extremely low and steam as a working fluid is in a state of wet steam that includes condensed fine droplets (droplet nuclei). The droplet nuclei condensed and deposited on a blade surface coalesce together to form a liquid film on the blade surface. The liquid film is torn off by steam of a working fluid main stream and sprayed downstream as coarse droplets, each droplet being considerably larger in size than the initial droplet nucleus. The coarse droplets, while being thereafter broken up into smaller sizes by the main stream steam, maintain certain sizes and flow downwardly. Unlike steam, the coarse droplets are unable to make a sharp turn along a flow path due to its inertia force and collide against a downstream moving blade at high speeds. This causes erosion in which the blade surface is eroded or impedes turbine blade rotation, resulting in loss.
To prevent, an erosive action by the erosion phenomenon, known arrangements are to coat a leading end of a moving blade leading edge with a shielding member formed from a hard, high-strength material such as Stellite. Alternatively, as disclosed in JP-UM-61-142102-A, one known method processes the surface of the leading edge portion of the blade to form a coarse surface with irregularities, thereby reducing an impact force upon collision of droplets with the blade.
It should, however, be noted that workability involved in each individual case does not always permit the mounting of the shielding member. Moreover, the mere protection of the blade surface is not generally a perfect measure against erosion and is typically combined with other erosion prevention measures.
Generally speaking, the most effective way to reduce effects of erosion is to remove the droplets. Exemplary methods in the above-described approach are disclosed in JP-1-110812-A and JP-11-336503-A, in which a hollow stationary blade has slits formed in its blade surface and the hollow stationary blade is decompressed to thereby suck a liquid film. The slits are very often machined directly in the blade surface of the stationary blade having a hollow structure. A still another method is as disclosed in J2-2007-23895-A, to machine an independent member that has a slit portion formed therein and to attach the independent member to the stationary blade.